Detoxification of hexavalent chromium containing coating on a metal surface



United States Patent US. Cl. 1486.21 5 Claims ABSTRACT OF THE DISCLOSUREA process for treating metal surfaces particularly aluminum and aluminumalloys, wherein the surface is treated with an aqueous acid solutioncontaining phosphate ions, fluoride ions, and hexavalent chromium ions,to produce a coating layer on the metal surface. Thereafter, the coatedsurfaces are treated with an aqueous acid solution which containssulfur-oxygen compounds, capable of reducing hexavalent chromium, whichsolutions also contain at least one element selected from the groupconsisting of beryllium, calcium, boron, aluminum, cerium, and thorium.The treatment with the latter solution effects a detoxification of thehexavalent chromium in the coating layer on the metal surface, thusavoiding the accumulation of toxic, chromic acid-containing wastesolutions. Desirably, the metal ions added to the acid treatingsolutions are present in amounts within the range of about 5 to 50milliequivalents per liter of the treating solutions.

This invention relates to a process for treating metal surfaces and moreparticularly it relates to the treatment of aluminum and aluminum alloysurfaces with a hexavalent chromium containing treating solution,wherein detoxification of the hexavalent chromium in the coatingproduced is effected.

In the chemical surface treatment of metals, particularly alumium andaluminum alloys, utilizing treating solutions which contain hexavalentchromium, rinse water is produced which contains appreciable quantitiesof chromic acid. In view of the extreme toxicity of chromic acid, itmust be rendered innocuous in this rinse water before disposal thereofcan be effected. Heretofore, this has been carried out by reducing thehexavalent chromium in the waste water to appreciably less toxictrivalent chromium salts, which salts may then, if desired, beprecipitated and removed from the water before it is disposed. Typicalreducing agents which have been used are sulfur dioxide, sodium sulfite,sodium bisulfite, sodium metabisulfite, and the like.

Moreover, processes involving so-called direct detoxification are alsoknown, wherein the accumulation of any appreciable amount of chromicacid-containing waste water is avoided. In these processes, theworkpieces, which have been coated with the solutions containinghexavalent chromium, are not rinsed with clean water but rather, arerinced directly, by spray or immersion, in a detoxification solutionwhich contains a suitable reducing agent for the hexavalent chromium,such as sulfurous acid or sodium bisulfite. In this manner, the coatingsolution adhering to the workpieces is uniformly detoxified and theworkpieces are thereafter rinsed in clean, flowing water. Inasmuch asthis latter rinsing removes only innocuous solution residues from thesurface of the workpieces, the resulting discharged rinse water issubstantially poison-free. Generally, in such a process, it ispreferable if the workpiece which is withdrawn from the coating solutiontreatment bath is permitted to drain prior to contacting it with thedetoxification solution. Additionally, if desired, an intermediate rinsein non-flowing water may be carried out prior to the application of thedetoxification solution. This non-flowing or static rinse water can thenbe used for replenishing the coating solutions. When the detoxificationsolution is consumed, it may then be discharged, preferably afterneutralization.

Although such processes of direct detoxification have proved to be aparticularly advantageous solution to the problem of toxic rinse watersfrom metal coatings containing hexavalent chromium, it has been foundthat in many instances, the use of such processes have a disadvantageousaffect on the chromate coating produced on the metal. In particular, ithas been found that the use of the acid detoxification solutionscontaining sulfur-oxygen compounds capable of reducing hexavalentchromium, such as sodium bisulfite, result in a decrease of the adherentstrength of the so-called green chromate coatings produced on the metal,as the metal surfaces are processed through the detoxification bath.Thus, the previously darkgreen colored coating layers become, at leastpartially, light green in color after use of the detoxification solutionand upon drying, the light green areas become powdery. Frequently, thesepowdery deposits can be easily wiped off the metal, down to the metalbase. This, of course, greatly reduces the effectiveness of thesematerials as protective and/ or paint-base coatings.

It is, therefore, an object of the present invention to provide animproved method for the direct detoxification of hexavalentchromium-containing coatings on metal surfaces, which method may becarried out without adverse affect on the coating.

A further object of the present invention is to provide an improvedmethod for coating metal surfaces with a hexavalent chromium-containingmaterial, wherein directdetoxification of the coating formed may beeffected without adverse affect on the coating.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows.

Pursuant to the above objects, the present invention includes a processfor the direct detoxification of a metal surface coated with ahexavalent chromium-containing coating which comprises contacting saidcoated surface with an aqueous acid treating solution containing atleast one sulfur-oxygen compound capable of reducing hexavalent chromiumand ions of at least one material selected from the group consisting ofberyllium, calcium, boron, aluminum, cerium, and thorium, andmaintaining said treating solution in contact with the coated metalsurface for a period sufficient to effect detoxification of the coating.In this manner, direct detoxification of the hexavalent chromium in thecoating on the metal surfaces is obtained with no deleterious affect onthe coating itself.

More particularly, in the practice of the present invention thedetoxification treating solution used are aqueous acidic solutionscontaining at least one sulfur-oxygen compound which is capable ofreducing hexavalent chromium. Exemplary of such compounds which may beused are sulfurous acid, hydrogen sulfite, as well as variouswater-soluble sulfites, disulfites, hydrosulfites (dithionites) andhyposulfites (thiosulfates), the various alkali metal compounds, ofthese, such as the sodium compounds, being particularly suitable.Desirably, the sulfur-oxygen compounds contained in the detoxificationtreating solution are present in amounts within the range of about 0.5to 3 grams per liter, calculated as NaHSO It will, of course, beappreciated that in many instances, amounts of the sulfur-oxygencompounds which are outside of this preferred range may also be used toobtain satisfactory results, the only requirement being that the amountsof these compounds used are sufficient to effect the desireddetoxification of the coating on the metal surface without adverseatfect on the coating itself.

In addition to the sulfur-oxygen compounds, the detoxification'treatingsolutions also contain one ion selected from the group consisting ofberyllium, calcium, boron, aluminum, cerium, and thorium. Desirably, theconcentration of these ions in the solution is within the range of about5 to 50 milliequivalents per liter, with amounts within the range ofabout 8 to 30 milliequivalents per liter being preferred. These ions maybe introduced into the treating solution in the form of various suitablecompounds which are soluble in the treating solution. Typical of thecompounds which may be used are nitrates, sulfates, hydroxides, and thelike. In many instances, treating solutions which contain aluminum ionshave been found to be particularly suitable, so that the addition ofsuch ions, desirably in the form of aluminum sulfate, is oftenpreferred, because of the particularly good activity obtained with thealuminum ions and the low cost and ready availability of the aluminumsalts, such as aluminum sulfate.

Inasmuch as the rate at which the hexavalent chromium is reduced by thetreating solution decreases greatly as the solution pH is raised aboveabout 6, it has been found to be desirable if the detoxificationtreating solution is utilized at a pH which is below about 6.Appropriate additions, as with acids or the like, may be made as arenecessary to adjust the pH into the desired operating range.Additionally, if desired, the treating solution may also contain asuitable surface active agent to aid in the drainage of thedetoxification treating solution from the metal workpieces, thusminimizing the amount of the solution which is carried out with theworkpieces.

In processing the hexavalent chromium coated metal surfaces with thedetoxification treating solution, the coated metal surfaces arecontacted with the solution using any suitable application technique,although contact by spraying with the treating solution or by immersingthe workpieces in the solution are preferred. The coated metal surfacesare maintained in contact with the treating solution for a periodSllfilClfiIlt to effect the desired reduction of the hexavalent chromiumin the coating. Typically, these contact times may be from a few secondsup to several minutes. Thereafter, if desired, the treated surfaces maybe rinsed in fresh water and then dried.

In the overall process for coating metal surfaces, utilizing the directdetoxification treatment of the metal, the metal surface, such asaluminum or aluminum alloys, are coated with a hexavalent chromiumcontaining coating solution. As is known in the art, the metal surfacesmay be subjected to various pretreatments, such as alkaline cleaning,etching, acidic passivation, and the like, prior to the application ofthe hexavalent chromium containing coating.

Various suitable hexavalent chromium solutions may be used, although thepresent direct detoxification process has been found to be particularlyapplicable on the socalled green chromate coatings. The coatingsolutions for producing such coatings generally contain phosphate ions,fluoride ions, and hexavalent chromium ions. Typical coating solutionsof this type are described in U.S. Patent 2,928,763 and may contain fromabout 5 to 150 grams per liter of phosphate ions, from about 2.5 to 62grams per liter of CrO from about 2.5 to 123 grams per liter of fluorideions. These coating solutions may be applied in any convenient manner,as is known to those in the art. Typically, the coatings are produced bycontacting the metal surface for periods of time from several seconds upto five or more minutes, with coating weights within the range of about10 to 1000 milligrams per square foot being typical of those produced,depending upon the particular coating conditions used.

Once the desired hexavalent chromium containing coating has beenproduced on the aluminum or aluminum alloy surface, the coated surfaceis then contacted with the detoxification treating solution, in themanner as has been described hereinabove. Rinsing of these surfaces,

following the treatment with the detoxification solutions, is found toproduce rinse water which is substantially free ;of toxic hexavalentchromium ions. Additionally, the :coatings on the metal surface arefound to be substantially unaffected by the detoxification solution,having good adhesion, with little or no lightening of the coating color,being evidenced.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. In these examples, unless otherwiseindicated, temperatures are in degrees centigrade and parts and percentsare by weight. It is to be appreciated, however, that these examples aremerely exemplary of the present invention and the manner in which it maybe practiced and are not to be taken as a limitation thereof.

EXAMPLE 1 An aqueous chromating solution was prepared containing thefollowing components in the amounts indicated:

Components: Grams per liter P 0 21.1 CrO 10.0 HF 2.72

Aluminum plates were degreased in a mildly alkaline cleaner, etched for2.5 minutes in a 5% aqueous sodium hydroxide solution at 50 degreescentigrade and rinsed with water. These plates were then passivated atroom temperature in a 15% aqueous nitric acid solution and again waterrinsed. The plates were then immersed for 2.5 minutes in the aboveformulated chromating solution, which solution was maintained at atemperature of 45 degrees centigrade. After their removal from thechromatin-g solution, some of the coated plates were rinsed with cleanwater while other plates were rinsed with various aqueous detoxificationsolutions, the rinsing being effected by immersing the plates for 30seconds in the solutions, which were at a temperature of 20 degreescentigrade. The aqueous detoxification solutions used contained 1 gramper liter of NaHSO and a 60 milliliters per liter of the aqueouschromating bath, as formulated above, to simulate an aged detoxificationsolution. Additionally, these solutions also contained variousadditives, indicated hereinafter, the amounts of the additives givencorresponding to 8 milliequivalents of the additives per liter of thedetoxification solution. Following the rinsing of the plates, witheither the clean water or the detoxification solution, the plates wereall again rinsed in water, allowed to drain and then dried for 8 minutesat C. The adhesive strength of the chromate layer on the plates was thendetermined by wiping the plates with a linen rag, using light pressure.The results thus obtained were reported as very low, indicating aremoval from 1 to 10% of the original layer thickness; low, indicating aremoval of from 11 to 30% of the original layer thickness; average,indicating a removal of from 31 to 70% of the original layer thickness;strong, indicating a removal of 71 to of the original layer thickness;and very strong, indicating a removal of 91 to of the original layerthickness. Using the above procedure, the detoxification solutions usedand the results obtained were as follows:

By way of comparison, the plates which had been rinsed only with cleanwater had a green-colored coating, the weight of which was about 5 gramsper square meter.

The adhesive strength of this coating layer was good, the wipe off beingvery low. Additionally, it was found that the rinse water from theseplates was quite high in the toxic CrO while the rinse water from theplates which had been rinsed with the various detoxification solutionscontained substantially no CrO EXAMPLE 2 The procedure of Example 1 wasrepeated with the exception that the additives in the aqueousdetoxification solution were as indicated below. Using this procedure,the following results were obtained:

As with the preceding example, although the adhesive strength of thechromate layer which had been washed only with water was good, with thewipe off being rated as very low, the water rinse solution obtained fromthis coating contained appreciable quantities of the toxic CrO Incontrast, the water rinse solution from the panels which had first beenrinsed with the detoxification solution contained substantially no CrOEXAMPLE 3 The procedure of Example 1 was repeated with the exceptionthat that additives for the detoxification solutions were as indicatedhereinbelow. Using this procedure, the following results were obtained:

pH Wipe ofi of Additive value the coating None 0.75 g./l. Be(NOa)2.3H Oplus 0.95 g./l.

2. 4 Very strong.

2. 5 Very low. C8(NO3)2.4H:O. 0.95 g./l. Ca(NOa)2.4-Hg0 plus 0.89 g./l.2. 2 Do.

A]2(SO4)3-18H20. 0.17 g./l. B(OH)3 Plus 0.89 g.ll. Al (SO4)a.18HgO 2. 2Do.

As with the preceding two examples, the chromate coating which had beenrinsed only with water showed good adhesive strength with a wipe offrating of very low. The rinse water obtained from the panel was,however, highly contaminated with appreciable quantities of CrO ascompared to the substantial complete absence of CrO in the rinse waterobtained from the panels which had been first rinsed with thedetoxification solution.

What is claimed is:

1. A method for treating metal surfaces which comprises contacting ametal surface coated with a hexavalent chromium containing coating withan aqueous acid treating solution containing at least one sulfur-oxygencompound which is capable of reducing hexavalent chromium in an amountwithin the range of about 0.05 to 3 grams per liter, calculated as NaHSOand ions of at least one material selected from the group consisting ofberyllium, calcium, boron, aluminum, cerium, and thorium, in an amountwithin the range of about 5 to 50 milliequivalents per liter of treatingsolution, and maintaining said aqueous acid treating solution in contactwith the coated surface for a period sufiicient to effect substantialreduction of the hexavalent chromium in the coating on said surface.

2. The method as claimed in claim 1 wherein the metal ion selected fromthe indicated group are aluminum ions.

3. A method of treating metal surfaces which comprises contacting themetal surface to be treated with a hexavalent chromium containingcoating solution, maintaining said coating solution in contact with themetal surface for a period sufiicient to form the desired hexavalentchromium containing coating thereon, containing at least onesulfur-oxygen compound capable of reducing hexavalent chromium in anamount within the range of about 0.05 to 3 grams per liter, calculatedas NaHSO and ions of at least one material selected from the groupconsisting of beryllium, calcium, boron, aluminum, cerium, and thorium,in an amount within the range of about .5 to 50 milliequivalents perliter of treating solution, and maintaining said treating solution incontact with the coated metal surface for a period sufficient to effectsubstantial reduction of the hexavalent chromium in the coating on themetal surface.

4. The method as claimed in claim 3 wherein the hexavalent chromiumcontaining solution is an aqueous acid solution containing phosphateions, fluoride ions, and hexavalent chromium ions.

5. The method as claimed in claim 3 wherein the ions selected from theindicated group in the treating solution are aluminum ions.

References Cited UNITED STATES PATENTS 3,391,032 7/1968 Hansen 148-6162,976,193 3/1961 Pimbley l486.21 2,854,371 9/1958 Schidderup 1486.21

ALFRED L. LEAVITT, Primary Examiner M. F. ESPOSITO, Assistant ExaminerU.S. Cl. X.R. 148-616, 6.2

